This disclosure relates to an electron beam vapor deposition apparatus and method for depositing coatings with desirable microstructural morphology and orientations on work pieces, and with improved thickness uniformity.
Electron beam vapor deposition devices are known and used for depositing coatings, such as ceramic coatings, onto work pieces. For instance, airfoils for use in turbine engines may include ceramic coatings to protect an underlying metallic alloy from oxidation and corrosion during engine operation. In some instances, there is a desire to form the coating with a columnar microstructure that is generally perpendicular to the underlying surface. The columnar structure increases the durability of the coating. However, available deposition processes are not practically economic or are incapable of depositing a suitable coating with the desired morphology, orientation, and thickness uniformity on all surfaces of a work piece.
Generally, electron beam physical vapor deposition (EB-PVD) involves using an electron beam to melt and evaporate a source coating material. The evaporated material condenses on the work piece. The energy state of the evaporated material, i.e., vapor, can be increased by increasing the electron beam power density in the molten pool of the source coating material.
With certain source coating materials, the temperatures of the underlying base alloy during coating required to ensure desirable columnar coating morphology are undesirably high, e.g., 2050 degrees F. Since the strength of the underlying base alloys are degraded by high temperature exposures, this results in an undesirably narrow processing window for the coating process. Increasing the energy state of the depositing molecules reduces the required temperature of the underlying base alloy during coating.
However, there is an upper limit to the beam power density delivered to the melt pool. Excessive beam power density can result in plasma formation in the coating chamber or in the electron beam guns. The plasma robs power from the beam, which can reduce power delivered into the melt pool. The pool may reduce temperature and may even freeze. When this happens, the cycle can start again resulting in a fluctuating process. The energy state of the vapor formed from the melt pool under these fluctuating conditions varies significantly, resulting in poor morphology of the coating, for some coating compositions. The vapor ions in the EB gun can cause plasma discharges and increased temperature inside the gun. What is needed is to enlarge the process parameter window while improving the process stability to improve the coating structure.